如何使用 Java 8 流迭代多级映射？

Question

我很擅长 Java 流，所以也许这不是最小的解决方案，但这是我想出的使用流来获得我认为你想要的东西的方法：

我们是否将 DishDiet 作为 POJO 并将 DishDietTest 作为主类？ 我正在根据需要获取数据，但如何循环它？ 如何使用 Java 8 流迭代多级映射？ 如何使用流完成基于分类的打印？

public class DishDiet {

    private final String name;
    private final boolean vegetarian;
    private final int calories;
    private final Type type;
    private final CaloricLevel caloricLevel;

    public DishDiet(String name, boolean vegetarian, int calories, Type type, CaloricLevel caloricLevel) {
        this.name = name;
        this.vegetarian = vegetarian;
        this.calories = calories;
        this.type = type;
        this.caloricLevel = caloricLevel;
    }

    public String getName() {
        return name;
    }

    public boolean isVegetarian() {
        return vegetarian;
    }

    public int getCalories() {
        return calories;
    }

    public Type getType() {
        return type;
    }

    public CaloricLevel getCaloricLevel() {
        return caloricLevel;
    }

    @Override
    public String toString() {
        return name;
    }

    
    public enum Type {
        MEAT, FISH, OTHER
    }

    public enum CaloricLevel {
        DIET, NORMAL, FAT
    }

}

public class DishDietTest {

    private static List<DishDiet> getAllDishDiet() {
        return Arrays.asList(new DishDiet("pork", false, 800, DishDiet.Type.MEAT, DishDiet.CaloricLevel.FAT),
                new DishDiet("beef", false, 700, DishDiet.Type.MEAT, DishDiet.CaloricLevel.FAT),
                new DishDiet("chicken", false, 400, DishDiet.Type.MEAT, DishDiet.CaloricLevel.DIET),
                new DishDiet("french fries", true, 530, DishDiet.Type.OTHER, DishDiet.CaloricLevel.NORMAL),
                new DishDiet("rice", true, 350, DishDiet.Type.OTHER, DishDiet.CaloricLevel.DIET),
                new DishDiet("season fruit", true, 120, DishDiet.Type.OTHER, DishDiet.CaloricLevel.DIET),
                new DishDiet("pizza", true, 550, DishDiet.Type.OTHER, DishDiet.CaloricLevel.NORMAL),
                new DishDiet("prawns", false, 300, DishDiet.Type.FISH, DishDiet.CaloricLevel.DIET),
                new DishDiet("salmon", false, 450, DishDiet.Type.FISH, DishDiet.CaloricLevel.NORMAL));
    }

    public static void main(String[] args) {

        Map<DishDiet.Type, Map<DishDiet.CaloricLevel, List<DishDiet>>> dishesByTypeCaloricLevel = getAllDishDiet()
                .stream().collect(groupingBy(DishDiet::getType, groupingBy((dish -> {
                    if (dish.getCalories() <= 400)
                        return DishDiet.CaloricLevel.DIET;
                    else if (dish.getCalories() <= 700)
                        return DishDiet.CaloricLevel.NORMAL;
                    else
                        return DishDiet.CaloricLevel.FAT;
                }

        ))));

        System.out.println(dishesByTypeCaloricLevel);

    }

}

Answer 1

你的意思是这样吗？

    dishesByTypeCaloricLevel.forEach((type, innerMap) 
            -> {
                System.out.println(type);
                innerMap.forEach((cl, list)
                        -> {
                            System.out.println("    " + cl);
                            list.forEach(dd -> System.out.println("        " + dd));
                        });
            });

输出：

OTHER
    NORMAL
        french fries
        pizza
    DIET
        rice
        season fruit
FISH
    NORMAL
        salmon
    DIET
        prawns
MEAT
    FAT
        pork
    NORMAL
        beef
    DIET
        chicken

我没有使用流进行迭代和打印。我建议我们只使用Map 和List 的forEach 方法。用法类似，但更简单一些。

我们是否将 DishDiet 作为 POJO 并将 DishDietTest 作为主类？ …

是的，没关系。

Answer 2

for(Map.Entry<DishDiet.Type, Map<DishDiet.CaloricLevel,List<DishDiet>>> t : dishesByTypeCaloricLevel.entrySet()){
    DishDiet.Type key = t.getKey();
    for (Map.Entry<DishDiet.CaloricLevel,List<DishDiet>> e : 
        t.getValue().entrySet())
        System.out.println("OuterKey:" + key + " InnerKey: " + e.getKey()+ " VALUE:" +e.getValue());
}